1. Field of the Invention
The present invention relates to an hydraulic drive system and in particular to a hydraulic drive controller for use with the hydraulic drive system for three wheel vehicles of the general type first described and claimed in our co-pending European Patent Application No. 97650025. (Specification No. EP0816153).
2. Background Information
In this European Patent Specification No. EP0816153 there is claimed a hydraulics drive system for a vehicle, having three independent wheel drive shaft assemblies in an essentially triangular configuration, namely a transversely in-line pair of wheel drive shaft assemblies and a single wheel drive shaft assembly longitudinally spaced from the in-line pair and, in which the drive system comprises a hydraulic pump and a drive unit directly coupled to each of the drive shaft assemblies namely a single drive unit and a pair of transverse drive units and in which each drive unit comprises at least two separate drives directly coupled to each drive shaft assembly, each drive having a separate hydraulic input and output and in which valving is provided to feed hydraulic fluid to each drive unit whereby the drive units operate in series or in parallel.
This is a particularly satisfactory method of operating the hydraulic drive system in that in almost all situations it eliminates the difficulties of skidding, wheel spin and the like because when using a series arrangement all the time a simple and effective drive system is provided. This is particularly effective for three-wheeled vehicles and for three wheeled vehicles of a relatively short wheel base. A problem arises when it becomes necessary to turn the vehicle when the drives are operating in series by the simple expedients of converting to parallel operation.
Previous attempts have been made to overcome the problems of the operation of three-wheeled vehicles such as, for example, by the use of an hydraulic drive system as described in French Patent Specification No. 2719001 (E.C.B. SARL). This patent specification acknowledges the need for safety reasons to ensure that the wheels are all synchronised and that if the distributional load is, for example, equally distributed over the three wheels, the hydraulic transmission operates relatively well but that on slopes and, indeed, in difficult terrain it can become dangerous to operate the vehicle. Further, this patent specification discloses the problems with the prior art in the use of flow dividers, electronic distributors and the like. This patent specification proposes an arrangement somewhat similar to that described in European Patent Specification No. 0816153, however, unfortunately a very elaborate desynchronisation unit comprising an electro-distributor which allows the flow of oil between the drives. However, with the electro-distributor which shut and if any of the wheels are locked or stopped for any reason with another wheel turning, excessive oil pressure can build up.
Certain problems have arisen in connection with the operation of a vehicle of with an hydraulic drive system in accordance with the invention described and claimed in this European Patent Specification No. EP 0816153 in the sense that it must always be appreciated that situations will arise, particularly when working in difficult terrain, when one of the wheels for example is stopped or turning more slowly than the others. While this does not generally happen due to the arrangement there are always situations when it may. This has unfortunately caused certain problems in operation.
The present invention is directed to overcoming these problems.
According to the invention there is provided an hydraulic drive system for a three wheeled vehicle, each wheel having a separate wheel drive shaft assembly, each comprising at least two separate drives directly coupled to each other and together forming an in-line pair of drive shaft assemblies and a single drive shaft assembly comprising:
an hydraulic pump;
an oil supply;
input feed piping connecting the hydraulic pump, oil supply and the drives; and
an hydraulic drive controller comprising:
hydraulic feed lines for feeding each of the drive units from the pumps;
hydraulic feed lines for return of hydraulic fluid from each drive unit to the pumps;
a secondary feed line connecting one of the drives of the single drive unit and one of the drives of one of the transverse drive units;
a further secondary feed line connecting the other drive of the single drive unit and one of the drives of the other drive unit;
a bypass hydraulic line mounted across each of the transverse drive units;
a diverter valve mounted in each bypass hydraulic line for parallel and series operation; and
a pressure release valve in each secondary feed line.
The advantage of this is that when, for example, one of the three wheels will not turn, in other words if it is jammed or stuck, the pressure in the secondary drive in the line may exceed maximum working pressure and this could cause serious problems in for example, a hose bursting. Many other operational conditions can cause the same problem.
Further the invention provides a drive controller in which the pressure relief valve directly feeds the oil supply sump.
In one embodiment of the invention the pump feeds oil through a non-return valve to each secondary hydraulic feed line. The advantage of this is that the non-return valve will prevent cavitation.
Further, the pressure relief and non-return valves are incorporated in the one assembly.
In another embodiment of the invention, there is a time delay between changing from series operation to parallel operation.
Preferably, the time delay is between 0.5 and 2.0 seconds.
Most preferably, the time delay is approximately 1.0 seconds.
In a further embodiment of the invention, each diverter valve is a hydraulic fluid operated diverter valve activated by a control valve in a hydraulic feed line of the drive controller and connected via a flow reducing device to the hydraulic oil supply, the control valve being movable from a position delivering pressurised fluid to the valve to a position closing off the supply of pressurised fluid and permitting flow of the hydraulic fluid out of the diverter valve through the flow reducing device to the sump.
Preferably, the control valve is a solenoid operated spool valve.
Most preferably, the diverter valve is a piston valve.
In another embodiment of the invention, the valves are housed in the one manifold block.